Oxygen bleaching of wood pulp, for example, is used as an extension to the kraft process to reduce the kappa no. and increase the brightness of the pulp and is generally followed by other bleaching stages such as peroxide and/or chlorine dioxide, ozone or the like. Oxygen bleaching is a cost efficient method of delignification because it uses very inexpensive chemicals. Thus, the lower one can reduce the lignin content of the pulp in an oxygen stage, the lower the bleaching costs can be, provided the characteristics of the resultant pulp meet the requirements of the customer.
In practice, oxygen is used only to a limited degree because of its lack of selectivity with respect to cellulose and the fact that it significantly lowers the viscosity of the pulp when used to produce a pulp with a low kappa number, i.e. approximately a kappa no. of 8 ml. The industry's response has been to extend pulping to lower kappa numbers by way of modified kraft pulping schemes and to limit the role of oxygen bleaching to remove only a modest amount of lignin.
Use of organic additives in the aqueous medium surrounding the pulp during the oxygen bleaching stage is known. Attention is directed to Japanese patent application 50-51889 published Mar. 2, 1993, issued to Mitsubishi Paper Mills Ltd., which discloses the use of minor amounts of nonionic surfactant and a derivative of ethylene diamine tetra acetic acid in an oxygen bleaching stage which the patentee claims, permits obtaining a lower kappa number pulp compared with the conventional medium concentration oxygen bleaching method.
Japanese patent 51-86987 published Jul. 27, 1993, issued to Sanyo Chemical Industries Ltd., teaches bleaching of the cellulose pulp with oxygen or peroxide in the presence of an ether compound, a polyol and an aliphatic monohydric alcohol. The ether compound may be derived from a polyhydric alcohol or its alkylene oxide adduct, preferably ethylene glycol. The amount of organic additive used is quite small and the effects obtained do not appear to be particularly significant, i.e. the viscosity obtained is very similar to the control at about the same kappa number.
Japanese patent 52-79979 published Oct. 26, 1993, issued to Mitsubishi Paper Mills Ltd., describes a bleached pulp obtained by bleaching with oxygen and a nonionic surfactants of polyether type compounds and may or may not include organic metal salt and glycol. The pulp produced is claimed to have the advantage of easier washing.